Eco-construction

Your reporter Fred Pearce has slipped from his normal standards of excellence (13 July, p 38). The magnesium-based cements that he describes are not a green panacea.

Pearce quotes John Harrison, a technologist from Hobart, Tasmania, who proposes an industry that takes magnesium carbonate ore and converts it into magnesium oxide cement. Buildings using the MgO cement would absorb carbon dioxide from the air, turning cities into CO2 sinks. But the cement can only reabsorb at most the same amount of CO2 after the structure has been built as was released during the manufacturing process. You can't use it to make a city a real carbon sink. The fact that magnesium cements are expensive should be a warning flag. It may just be due to a lack of economies of scale, but it's equally likely to be due to the nature of the whole industry, from ore extraction, refining, ...

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